This invention relates to a gas detection apparatus, e.g., an apparatus for detecting a gas constituent in the exhaust gas of an engine for the purpose of enhancing the rate of purification of a catalytic converter for purifying the exhaust gas.
Conventionally as an apparatus for detecting a gas constituent in the exhaust gas of an engine there has been proposed an apparatus which comprises a gas detecting element of a metal oxide semiconductor such as titanium oxide whose electric resistance depends on a relative gas constituent and a comparator which converts the resistance of the gas detection element to a voltage by connecting the element in series with a fixed reference voltage and compares the voltage with a reference voltage in order to give an output signal indicative of a gas constituent.
However, the above prior art apparatus has only one reference voltage for comparison and the reference voltage is fixed. Therefore, if the electric resistance of the gas detection element changes in accordance with the working temperature or with the lapse of time, an erroneous detection of gas constituents may occur or it may become impossible to make detection.